Recently, together with the trend of mobile communications terminals becoming multi-functional and multi-banded, the number of components that constitute a so-called RF (Radio Frequency) front end tends to be increasing. On the other hand, there have been demands for keeping the size and weight of a mobile communications terminal small.
Further, among the components of a mobile communications terminal, filters are frequently used at an RF stage and an IF (Intermediate Frequency) stage. These filters should meet the requirements of low loss, good out-of-band attenuation characteristics and wide bandwidth.
In a mobile communications terminal, a duplexer is a component installed directly under an antenna.
The duplexer is formed by connecting a plurality of filter having different frequency passbands. To meet the demands for reducing the size of devices, the duplexer is recently manufactured by using a surface acoustic wave filter.
Further, a surface acoustic wave duplexer was proposed to improve isolation characteristics while suppressing an insertion loss, in which a resonator at a first stage of a transmitting filter formed of a ladder filter is configured as a parallel resonator, and its capacitance is set to be smaller than half of those of other resonators in parallel arms.